1. Field of the Invention
The present invention relates to a method and apparatus in which ultrasonic waves are used to determine temperature changes inside tissue in vivo by utilizing the temperature dependency of the acoustic characteristics of the tissues as obtained by, for example, transmitting ultrasonic waves into the tissue and receiving the reflections of these waves.
2. Prior Art Statement
Ultrasonic diagnostic devices are known which perform measurements in vivo by means of ultrasonic waves. Most of these ultrasonic diagnostic devices employ the pulse reflection method in which the measurements are performed by transmitting ultrasonic pulses into the living tissue and using information obtained from the pulses reflected back from the tissue. In the pulse reflection method, internal information on the tissue comprised of the strength, i.e., the amplitude, of the reflections from boundaries in the tissue where there are changes in acoustic impedances and the propagation delay times is gathered two-dimensionally to provide a tomographic image. However, in recent years there has been increasing demand for ultrasonic diagnostic apparatuses which can be used to obtain other information in addition to that relating to the shape of in vivo tissues. One example of such information is the internal temperature of the tissue. The ability to obtain internal temperature information in vivo would make it possible to monitor the temperature during cancer thermotherapy. Temperature can be deduced in vivo by measuring acoustic characteristics such as, for example, ultrasonic wave attenuation, sonic velocity, the non-linearity parameter B/A and the like, followed by comparison of the measured values with those obtained through prior investigation of the temperature dependency characteristics of such acoustic characteristics. A method that is known relating to obtaining information relating to the non-linearity parameter was disclosed in Japanese Laid-open patent publication No. 60(1985)-119926, and will be described briefly here.
This method uses the non-linearity of the dependence of the sonic propagation velocity on the wave particle velocity or sound pressure. Because of this, transmitted into the tissue are a relatively high-frequency probe pulse from a transmit/receive oscillator, and a relatively low-frequency pump pulse from a location substantially the same as that of the probe pulse and in the same direction. The drive timing of the probe pulse oscillator and the pump pulse oscillator is adjusted so that the measurement probe wave is superposed on the pump wave at the positive particle velocity portion, as shown in FIG. 9 (a) (or at the negative particle velocity portion, as in FIG. 9 (b)). By then obtaining the difference in phase between the reflected probe pulse signal received when both pump pulse and probe pulse are transmitted into the tissue, and the phase of the reflected probe pulse signal received when only the measurement pulse is transmitted, or of the signal received when the transmission is of both pump pulse and probe pulse, arranged so that the phase is reversed compared with the initial transmission, the phase modulation of the measurement pulse arising from the effect of the pump wave alone is detected using the reflected pulse method, to thereby obtain the acoustic non-linearity parameter B/A of the tissue. Thus, when attention is focused on the progress of the probe pulse, the probe pulse is phase modulated by an amount decided by the traversed-distance integral of the product of the non-linearity parameter of the region traversed by the probe pulse until reaching a reflector (a position function), and the pulse wave amplitude. Utilizing this, the signals being reflected back from different depths are received and demodulated and the changes in the phase signals thus obtained are found, and depthwise differentiation is also used to obtain the distribution of the non-linearity parameter B/A.
In the said conventional non-linearity parameter measurement method, however, because the probe pulse superposition is at the particle velocity positive (or negative) peak portions of the pump pulse, at instants in which the particle velocities of both pulses which are on the increase are compounded, the velocity becomes very high, which produces abnormal distortion in the probe pulse and adversely affects the measurement, and there is also a risk to the safety of the living body. In addition, the only acoustic characteristics obtained are those relating to the non-linearity parameter, with no provision to simultaneously obtain other information, for example attenuation characteristics, and as such it is impossible to measure temperature with high reliability.
The present inventors have also proposed (Ref. U.S. Pat. No. 4,754,760) a method of obtaining the internal temperature. This method consists of transmitting pump pulses and probe pulses into a specimen, obtaining the variations in delay times from non-linearity interactions and the crossover frequencies of the spectral distributions of the received signals corresponding to the respective phase states of the pump pulses and probe pulses produced in the specimen. By also varying the strengths of a multiplicity of pump pulses to change the delay time variations and finding the crossover frequencies, the center frequency of the spectral distributions of the received signals can be found with good precision. The ultrasonic wave attenuation characteristics can then be obtained from changes in these center frequencies at different depths, and the amount of temperature change found from variations in the ultrasonic wave attenuation characteristics.